1. Field of the Invention
The present invention relates to a mechanism for automatically adjusting the tension of a belt as a winding and transmitting device for automatic door wound around a driving side transmission wheel (sprocket, etc.) and a driven side transmission wheel (pulley, etc.)
2. Description of the Prior Art
As illustrated in FIG. 8, an automatic door comprises a driving side transmission wheel P.sub.1 connected to a motor M, a driven side transmission wheel P.sub.2 corresponding thereto, a belt V such as a V belt wound around the transmission wheels described above, a rail R, a traveling body C supported on the rail R, and door D mounted on the traveling wheel C to which the above described belt V is connected.
A belt V of such an automatic door is elongated with the lapse of time even if it is strained at first. In addition, there may be a case where the belt V is temporarily elongated upon starting or stopping the door D. Such elongation of the belt makes it impossible to smoothly open and close the door D. Accordingly, there have been proposed various mechanisms for allowing looseness of the belt following the elongation thereof to be automatically compensated for.
FIG. 9 shows a prior automatic tension adjusting mechanism disclosed in Utility Model Gazette No. 35644/78. The mechanism includes a base 30 fixed in a casing, a mount 31 for a transmission wheel P.sub.2 around which a belt V is wound, a guide lever 32 for guiding a movement of the mount 31 so as to make the mount 31 slidable in the right and left directions where the mount forces the belt V to be strained and relaxed, a spring 33 for resiliently energizing the mount 31 at all times in a direction where the mount 31 forces the belt V to be strained, a stopper part 34 having a plurality of grooves in the base 30, a pawl 35 provided at an end part of the mount 31 and detachably fitted in the groove formed in the stopper part 34, and a tapped hole made in the mount 31 into which a bolt 36 is screwed.
A groove shape of the stopper part 34 is such specified that the mount 31 can be moved to the left in the figure when the pawl 35 remains fitted in the groove of the stopper part 34, but not moved to the right. Accordingly, with the bolt 36 loosened, the mount 31 is moved in the left direction in the figure due to resilient energization by the spring 33. This is a direction where the belt V is strained, so that the belt V can be prevented from being relaxed when it is elongated due to various causes. Further, provided that the pawl 35 of the mount 31 is lifted out of the groove formed in the stopper part 34 by tightening the bolt 36 to push against the base 30, the mount 31 can be moved to the right resisting to the energization by the spring 33 as shown by a virtual line in the figure. It is facilitated in the situation to wind the belt V around the transmission wheel P or remove it therefrom.
However, although in the prior automatic tension adjusting mechanism described above the mount 31 can be moved in the right direction when the bolt 36 has been tightened, it can not be fixed at an end point of the movement since the resilient energizing force by the spring 33 is always applied thereto. Thus, some locking mechanism is needed to achieve it. Namely, such a difficulty that the mount 31 can not be fixed at the end point of the movement thereof originates in that the prior mechanism illustrated in FIG. 9 has not such a locking mechanism.